Start wth lunches and coffee breaks -- be too busy to go on your regular "dates" and start lunching with others. Ask your boss for a new project or just start something you think will be useful and use it as an excuse to keep working through lunch.

Stop all non-work related texts and messaging. Never, answer any out of work message. Leave any she sends outside of work unread. Avoid all personal topics in your discussion with her. That means nothing about some interest you know you share, no consoling her about her personal problems and not sharing yours. When you do talk, mention plans for an outing with your girlfriend and be intentional about planning things for your girlfriend. If this woman tries to rein you back in, reject her firmly. Under no circumstances you confess feeling or talk about "if I were not...". If necessary, seek a transfer to a different department